1. Field of the Invention
The present invention relates to the field of tamoxifen derivatives. In that novel tamoxifen derivatives are described wherein the alkyl chain of the molecule is substituted with fluorine or iodine, the present invention also relates to methods of synthesizing the novel fluoro- and iodo-tamoxifen analogs and derivatives. In that the described tamoxifen derivatives have high affinity for binding estrogen receptors and may be labeled with detectable "tagging" molecules, rendering highly visible labeled estrogen receptors through position emission topography (PET) and single proton emission tomography (SPECT), the present invention also relates to the field of Positron Emission Topography and single photon Emission Tomography reagents and radiopharmaceuticals. The haloginated tamoxifen derivatives of the present invention are advantageously used in the imaging of estrogen receptors, for example, in breast, ovarian, uterine and brain tissue.
The present invention also relates to the field of anti-cancer therapeutic agents, particularly to methods of breast tumor therapy and diagnosis, in that the described high estrogen receptor binding affinity iodo- and fluoro- tamoxifen analogues may be advantageously used in the therapy and diagnosis of estrogen-receptor positive breast cancers.
2. Background of the Invention
Endocrine therapy is one of the oldest nonsurgical methods for treatment of breast carcinoma, and is still considered standard for certain subsets of patients, typically postmenopausal women with primary tumors and tumors having high estrogen levels..sup. 1-3 The synthesis of F-18 fluoroestradiol for application in diagnosing breast tumors in humans has recently been described..sup.4 Observation of significant changes in the binding of estrogen receptors in breast tumors were reported using PET. However, technical difficulties associated with estrogen receptor saturation in patients receiving tamoxifen, or other estrogen receptor antagonist, has been observed to decrease the sensitivity and accuracy of using an estrogen-based receptor tag in diagnosing and monitoring the progress of tumors in patients receiving such treatments.
Tamoxifen (I), a potent non-steroidal anti-estrogen, has been widely used in the treatment of human breast tumors. Tamoxifen has few side effects when compared with other hormonal treatments. Tamoxifen is cytostatic (i.e, it prevents/inhibits cell growth), and exerts competitive inhibitory activity at the receptor level with estrogen. More specifically, the cytostatic activity of tamoxifen is described from its action to bind to cytoplasmic estrogen receptors and is translocated to cell nuclei, where cell proliferation is prevented..sup.1-3 Thus, tamoxifen is often administered as an anti-cancer agent..sup.6 For example, Foster et al. describes the effect of various tamoxifen hydroxiyderivatives on the growth of MCF-7 breast cancer cell line (Id.) in its native form. However, highly active in vitro hydroxy tamoxifen derivatives were found to be less active than tamoxifen in vivo against a DMBA-induced ER-positive tumor in rats and only slightly more active against a hormone dependent mammary tumor in mice.
Tamoxifen has a relatively low binding affinity for the estrogen receptor (ER). Attempts have therefore been made to synthesize tamoxifen derivatives having improved ER binding affinity to enhance to action as an anti-cancer therapeutic agent. A variety of these tamoxifen derivatives have been described in the literature. Structural modifications have been made at virtually every site on the three aromatic rings of the tamoxifen molecule. The structure of tamoxifen is demonstrated as: ##STR1##
For example, 4-hydroxy tamoxifen derivative in which X=--OH, was shown to be a potent anti-estrogen, but proved in vivo to be less effective than tamoxifen, owing to rapid glucuronidation of the hydroxyl group followed by excretion. ##STR2##
Other tamoxifen derivatives having a 4-position substitution of the phenyl ring, in which X is methoxy, methyl, fluoro or chloro, have also been proposed and evaluated..sup.15 K. E. Allen et al. (1980) conducted studies wherein the 4- methyl, chloro- and fluoro-derivatives were evaluated and found to have approximately equiactivty for estrogen receptor affinity compared to tamoxifen in vitro. However, in vitro uterine weight tests indicated that these derivatives had lower anti-estrogenic activity than tamoxifen, while other tests indicated that the activity of the 4- methoxy derivative was about the same as tamoxifen.
A 4- iodo substitution of the phenyl ring as a tamoxifen derivative (formula 2: X=iodo) has recently been found to have greater potency than tamoxifen in relation to detecting estrogen receptor-positive breast cancer..sup.13 Other 3- and 4- iodo and -bromo-phenyl ring substituted tamoxifen derivatives- have also been described..sup.13 For example, the McCaque et al. patent (U.S. Pat. No. 4,839,155), described the preparation of a halogenated tamoxifen. The halogen atom (i.e., I or Br) was substituted at one of the phenyl rings of the tamoxifen structure.
However, derivatives of tamoxifen wherein sites other than those on the phenyl groups of the molecule are substituted have not been proposed in the art. Such a molecule would be desirable, as it would leave the major portion of the molecule unchanged and free to bind with the "target" molecule or tissue cells. Additionally, to further enhance tissue targeting specificity, the tamoxifen molecule would preferably be coupled with a targeting molecule such as a microparticle. Such a tamoxifen derivative has not as yet been developed, and would represent a tremendous improvement in the quality of imaging techniques currently available, as well as improve the quality and accuracy of PET and SPECT scans.
Other alternative compounds proposed as a possible steroid radiopharmaceuticals useful in the imaging of steroid receptors include labeled progesterone and estrogen derivatives. For example, Pomper et al. described a ligand for the progesterone receptor..sup.16 This aliphatic fluorination of FENP (21-[.sup.18 F] fluoro-16 .alpha.-ethyl-19- nor progesterone) is described as demonstrating a high specific uterine target tissue uptake..sup.16 This ligand for the progesterone receptor is labeled with the position-emitting radionucleotide fluorine-18 (t 1/2=110 min). Estrogen-based imaging agents described in the literature include radionucleotides of iodine.sup.20, fluorine.sup.19, and bromine.sup.21. By way of example, a specific fluorine described in the 16 .alpha.-[.sup.18 F] fluorolstradiol ligand..sup.17
The preparation of 16 .alpha.-[.sup.18 F] fluoroestrogens and their selective uptake by estrogen target tissues in rats has been described by Kiesewetter et al..sup.19. Additionally, significant changes in the binding of estrogen receptors in breast tumor were reported with the use of [.sup.18 F] fluoroestradiol using PET..sup.4 However, the radioisotope .sup.18 F has a very short half life, and therefore techniques and molecules which employ this radioisotope must be rapid, and preferably more rapid than currently employed molecule labeling techniques allow.
Additionally, the use of an estrogen-based agent in response to hormone therapy with estrogen receptor positivity may not closely correlate actual physiological response to the hormonal therapy. A progestin-based imaging agent for breast tumors might be preferred over an estrogen-based agent because response to hormonal therapy is more significantly correlated with progesterone receptor positivity than with estrogen receptor positivity..sup.17 It has further been reported that estrogen receptor positive tumors in patients on hormonal therapy (e.g. tamoxifen) could not be imaged with an estrogen, as the circulating levels of tamoxifen and its metabolites are sufficiently high to fully occupy the estrogen receptor..sup.18
While the tamoxifen derivatives described in the literature have demonstrated some increase in estrogen receptor binding affinity, they do not demonstrate sufficient specific radioactivity due to the fluorine modification of those molecules at sites on the phenolic rings. Thus, the enhanced estrogen receptor binding affinity is achieved at the expense of a loss in specific activity. The fluorine-18 [.sup.18 F] ion radioisotope reportedly has a low effective dose equivalency and a short half-life (+1/2=110 min). For these reasons, methods which require [.sup.18 F] in labeling the tamoxifen molecule must be rapid (i.e. within a 2 hour reaction time) to avoid a loss in activity of the label.
Tamoxifen derivatives substituted at various phenolic sites of the tamoxifen structure may block the active hydroxy metabolite formation. This may decrease the tamoxifen analogs potency and receptor binding affinity. On the other hand, an elimination reaction may occur due to competition with active metabloite formation on the phenyl ring. A tamoxifen derivative which therefore was instead substituted at other sites on the molecule would advantageously provide a molecule with an endhanced in vivo stability and estrogen receptor binding capacity than currently available derivatives and analogs.